At first glance, solar sails look like vast solar panels designed to power spacecraft but they actually work very differently.
Many probes rely on solar panels to tap into solar energy, converting sunlight into power for their onboard computers. But solar sails work like the sails on a yacht, except they’re pushed along by “solar wind” – the constant stream of photons coming from the sun.
On Earth you don’t feel the minute force that sunlight exerts on your skin, but solar wind is actually a significant force. It’s the reason why a comet’s tail always points away from the sun.
Unfurling a solar sail doesn’t offer the same kick as firing up a rocket, but the benefit of a solar sail is that it doesn’t require any fuel. As long as the sun keeps shining, the spacecraft will keep sailing forever. You still need a traditional rocket to break the Earth’s bonds and lift the craft into space, but launches are cheaper because the solar sail spacecraft isn’t carrying heavy rocket fuel.
Solar sails are already in action, carrying Japan’s IKAROS 2010 probe to Venus and then into orbit around the sun. NASA has also tested solar sails in low Earth orbit, with the NanoSail-D mission exploring the feasibility of using solar sails to shift defunct satellites out of orbit.
This year the Planetary Society – a nonprofit advocacy group for space exploration backed by Bill Nye “The Science Guy” – ran a Kickstarter campaign to help fund its LightSail project and secured more than four times its $200,000 goal.
The LightSail has seen its first successful test, using a 32-metre wide solar sail to orbit the Earth for several days before burning up in the atmosphere. The tiny LightSail-A craft was funded entirely by public donations – and will be followed by LightSail-B next year.
Craft powered by solar sails accelerate slowly but, as they never run out of fuel, they can eventually reach greater speeds than craft relying on traditional rockets. Solar sail missions to the outer planets may even slingshot around the sun to build up speed.
As for the future, it’s feasible that giant solar sails could eventually be used for interstellar travel, but it would still take thousands of years to reach the nearest star (which is four light-years away).
While solar sails can propel a craft, they can’t power the onboard systems. But many space missions still rely on solar panels and rechargeable batteries: The International Space Station, as an example, relies on eight solar arrays with a wingspan wider than a Boeing 777 jumbo jet. The giant array rotates to always face the sun.
Meanwhile, the tiny Mars rovers Spirit and Opportunity rely on solar panels to charge their lithium ion batteries (an interstellar version of the batteries that power houses). Deployed for a three-month primary mission in 2003, both rovers far exceeded expectations and Opportunity continues to return data from the red planet.
So the next time you look at your solar panels, remember the difference they’re making to your home, but also thousands of kilometres above you.
The future is as bright as the sun that will power it.